In a hydraulic axial piston pump, a component force, i.e. a lateral force, at right angles to the piston axis acts on the piston as a reactive force according to the inclination of a swash plate or a cylinder block. Therefore, a large frictional force is produced on the sliding surfaces of the piston and the cylinder bore.
When oil is used as the working fluid, it lubricates against the friction of the sliding surfaces, and it therefore provides durability.
However when water is used, lubricating performance is low, and durability remarkably decreases.
Attempts have been made to lubricate the sliding surfaces with lubricating oil and to prevent the oil from mixing with water by a seal provided on the outer circumference of the piston, but as the seal is not perfect, the water is polluted by the oil.
In Japanese Utility Model Laid-Open 48-55229, 48-6824, 48-57702, 48-68203 or Japanese Patent Laid-Open 8-151975 disclosed by the inventor, a construction was proposed wherein the piston and a shoe are brought into contact on a flat surface at right angles to the piston axis to decrease the lateral force acting on the piston. Therefore, component force acting in a direction at right angles to the piston axis which is exerted by the shoe on the piston does not occur, and the lateral force acting on the piston is very much reduced.
The friction of the sliding surface between the piston and the cylinder bore is thereby decreased, but as lubricating performance is poor when water is used as working fluid, there is still a large friction on the sliding surface not only between the piston and the cylinder, but also between the piston and shoe or between the shoe and swash plate. There was thus still a problem of durability.
This problem was not solved by the pumps disclosed in the specifications of German Patents 529589, 597476, and U.S. Pat. No. 3,162,142.